1. Field of the Invention
This invention relates in general to a circuit for receiving digital intelligence signals in a digital switching center of a time-division multiplex communication network wherein the signals are transmitted on extension circuits from digital subscriber stations and the signals in the form of an intelligence signal block containing at least one intelligence word and if necessary a signalling word as well as a synchronizing word which precedes the other portions.
The bit pulses of intelligence signals is primarily synchronous with the exchange pulse of the switching center since the clock pulse generators of the individual subscriber stations are synchronized with the exchange pulse. However, because of the varying transit times of the individual extension circuits which can also be subject to different fluctuations and interference the time of reception at the switching center cannot be precisely fixed. Thus, the circuit arrangement for receiving the intelligence signal blocks in the switching center must be able to accept the intelligence signal blocks in proper phase at any time within a specified reception time span. Also, the switching center must release the information for further processing only if the information includes a proper synchronizing word. Also, the circuit arrangement must not incorrectly interpret interferring signals that occur within the reception time span but displaced with respect to the intelligence signal blocks so that such interferring signals are not evaluated.
2. Description of the Prior Art
A proposed circuit arrangement is illustrated in German Patent Application No. P 28 32 589.5 discloses a circuit wherein a code representation of the digital intelligence signals can be distinguished from periods in which no intelligence signals occur on the extension circuit. In this system, a reception control signal can be derived from the intelligence signal blocks received which indicates its position and length.
However, such preconditions are not always given or, respectively, the creation of such conditions requires additional equipment outlay.